The exhaust purification system for an internal combustion engine purifies HC (hydrocarbons), CO (carbon monoxide) and NOx (nitrogen oxides) contained in the exhaust gas of the engine. For exhaust purification systems, those using reactions on various types of catalysts provided in the exhaust passage to purify the above-mentioned three-way components in exhaust gas have become mainstream. In the catalysts purifying the exhaust gas, various catalyst have been proposed that have different functions such as an oxidation catalyst (DOC (Diesel Oxidation Catalyst)), three-way catalyst (TWC (Three-Way Catalyst)), NOx storage reduction-type catalyst (NSC (NOx Storage Catalyst)), and selective reduction catalyst (SCR catalyst (Selective Catalytic Reduction Catalyst)).
The oxidation catalyst has an oxidizing function for purifying HC and CO, by causing the oxidation reaction of HC and CO to progress under exhaust gas with the equivalence ratio of the air/fuel mixture made lean to abundantly contain oxygen (exhaust gas of lean equivalence ratio). In addition, this oxidation catalyst also includes a three-way purification function whereby the oxidation reaction of HC and CO and the reduction reaction of NOx simultaneously progress at high efficiency under exhaust gas with the equivalence ratio of the air-fuel mixture made stoichiometric (exhaust gas of stoichiometric equivalence ratio). The three-way catalyst corresponds to a catalyst made by adding an oxygen storage material (OSC material) to the above-mentioned oxidation catalyst, and when comparing with the above-mentioned oxidation catalyst, the three-way purification window, i.e. the equivalence ratio width exhibiting the three-way purification function, becomes wider. This effect occurs from the width of the catalyst internal air-fuel ratio fluctuation relative to the fluctuation of the pre-catalyst air-fuel ratio decreasing by way of the oxygen storage effect of the OSC material.
The selective reduction catalyst reduces NOx under the presence of a reducing agent supplied externally such as NH3 or HC, or existing in the exhaust gas. The NOx storage reduction-type catalyst stores NOx in the exhaust gas under exhaust gas of lean equivalence ratio, and reduces the NOx stored under the exhaust gas of stoichiometric or richer than stoichiometric equivalence ratio by way of the reducing agent. The exhaust purification system of an engine based on lean combustion, such as a lean combustion-type gasoline engine or diesel engine, often uses a catalyst called a DeNOx catalyst such as this selective reduction catalyst or NOx storage reduction-type catalyst, in order to ensure the NOx purification performance under exhaust gas of lean equivalence ratio, by combining with the aforementioned such oxidation catalyst or three-way catalyst.
Patent Document 1 presents an exhaust purification system combining, among the above such catalysts, the NOx storage reduction catalyst and three-way catalyst. This exhaust purification system sets the equivalence ratio of the air-fuel mixture to stoichiometric prior to the NOx storage reduction-type catalyst reaching activation, and purifies the three-way components of exhaust gas by way of the three-way catalyst mainly. In addition, this exhaust purification system sets the equivalence ratio of the air-fuel mixture to lean after the NOx storage reduction-type catalyst has reached activation, and purifies HC and CO with the three-way catalyst, as well as purifying NOx by way of the NOx storage reduction-type catalyst.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2009-293585
According to the above such exhaust purification system of Patent Document 1, it is possible to purify the three-way components in the exhaust gas both during lean operation controlling the equivalence ratio of the air-fuel mixture to lean and during stoichiometric operation controlling the equivalence ratio of the air-fuel mixture to stoichiometric. However, with the system using the three-way purification function intermittently in this way, performing equivalence ratio control precisely becomes important particularly during stoichiometric operation, as explained below. However, this point is not sufficiently considered in Patent Document 1.
First, when supposing not possible to precisely control the equivalence ratio of the air-fuel mixture to stoichiometric, a three-way catalyst containing a sufficient amount of OSC material and including a sufficiently wide purification window must be used in the catalyst for exhibiting the three-way purification function. In addition, when the content of OSC material increases in this way, not only does the cost increase proportionally, but also the adverse effect also arises in that the oxidation performance for HC or CO declining under exhaust gas of lean equivalence ratio. Furthermore, when increasing the content of OSC material, since the oxygen stored on the OSC material during lean operation also comes to be abundant, the time required until the OSC material finishes releasing oxygen when changing the pre-catalyst air-fuel ratio from lean to stoichiometric, i.e. time required until the air-fuel ratio environment on the catalyst switches from lean to stoichiometric, lengthens, and thus the adverse effect also arises in that the required time until the NOx purification rate rises lengthens. It should be noted that, although it has been considered to richen the air-fuel ratio in order to shorten the oxygen release time of the OSC material, the emission amount of HC and CO to downstream of the catalyst increases in this case.
In addition, for a DeNOx catalyst such as a general NOx storage reduction-type catalyst or selective reduction catalyst, the NOx purification performance declines during high-load operation such that the exhaust gas volume increases and the exhaust gas temperature rises. For this reason, it has been considered to perform stoichiometric operation also during high-load operation, and use the three-way purification function of the three-way catalyst under exhaust gas of stoichiometric equivalence ratio to compensate for the decline in purification performance of the DeNOx catalyst. However, during such high-load operation, since the purification window of three-way catalyst also narrows, high-precision equivalence ratio control is still necessary in order to exhibit sufficient three-way purification performance.
The present invention has been made by considering the above such points, and has an object of providing a control device for an internal combustion engine capable of high-precision equivalence ratio control.